Wireless communications in millimeter wavelength frequency bands, such as the E-band (71-76 GHz and 81-86 GHz), typically have data rates in the order of Giga bits per second (Gbps). At such high data rates, mitigating Intersymbol Interference (ISI) caused by radio signal multipath propagation in a wireless channel, and signal reflection caused by connecting cables, is always a significant technical challenge.
Orthogonal frequency division multiplexing (OFDM) and its variants, such as single carrier with frequency domain equalization (SC-FDE), typically cope with large multipath delay spreads in broadband communications. However, equalization of OFDM signals in the frequency domain introduces large processing delays, and the spectrum efficiency is also reduced due to the use of guard intervals.
A single carrier system with advanced equalizers, such as a decision-feedback equalizer, is another option for coping with ISI. However, for high speed systems which demand very high clock rates in firmware implementations, such equalization cannot be performed at sufficiently high speeds to satisfy the data rate requirements. Therefore, a single carrier system with linear equalization becomes the only viable solution to ISI mitigation for high speed systems, when low processing delay is demanded.
Transmitter side equalization, which is referred to as pre-equalization hereafter, is efficient in reducing the implementation complexity and noise enhancement effect associated with receiver side linear equalization. Generally, a linear equalizer needs to have a long impulse response to equalize a linear channel with even a short delay spread, which implies that the equalization complexity will generally be very high if the equalization is implemented at the receiver. Such equalization can also cause a significant noise enhancement effect. Shifting such equalization from the receiver to the transmitter (i.e., pre-equalization) can significantly reduce the implementation complexity and latency by using predefined lookup tables created based on a pre-defined signal constellation. The noise enhancement effect can also be mitigated as the signal to noise ratio at the transmitter is much larger compared to that at the receiver.
More generally, both pre-equalization at the transmitter side and equalization at the receiver side are implemented at the same time. In one approach, the impulse response of the communications channel is factorized as a product of two impulse responses, and each is compensated for by either transmitter or receiver equalization. However, very complex computations are required for such factorization. In another aspect, channel equalization is mainly implemented at the transmitter, and receiver side equalization is only used to deal with residual channel effects after pre-equalization at the transmitter.
Coefficients used in the pre-equalization at the transmitter need to be generated using, for example, the impulse response of the channel estimated at the receiver. However, when channels are time varying, a mechanism is required to track the channel variation and update the equalization coefficients. Typically, pre-equalized or non-pre-equalized training sequences are used for estimating the impulse response of the channel, and generating the equalization coefficients at both transmitter and receiver.
The impulse response of the channel is typically estimated in the frequency domain due to its low complexity. When a pre-equalized training sequence is used, in the frequency domain, the received signal at one frequency point can be represented as y=hpx+n, where y is the received signal, h is the channel response, p is the pre-equalizer coefficient, x is the training signal, and n is the noise. The receiver equalizer coefficient can be generated by treating hp as a combined channel response, while the transmitter pre-equalization coefficient needs to be determined through the channel response h, which can be obtained by removing the pre-equalizer coefficient p from the estimate of the combined channel response hp. When non-pre-equalized training sequences are used, the received signal is y=hx+n, and the estimation of the receiver side equalization coefficient needs to combine the pre-equalizer p with the estimate of the channel response h. In either case, the receiver needs to know when the pre-equalization coefficients are updated. Estimation performance is also affected by using non-constant magnitude training signals in the frequency domain in the case of using pre-equalized training and by a doubled noise effect by combining the estimate of the impulse response of a noisy channel and the pre-equalizer in the case of using the non-pre-equalized training sequence.
In existing systems, these training sequences in every frame are generally identical. If multiple training sequences are required, they are concatenated in the preamble of a frame. However, long preamble causes long delay.
In-phase and quadrature (I/Q) imbalance is another significant concern for a wireless system with I/Q modulation architecture, i.e., the baseband signal is modulated onto (or demodulated from) an intermediate frequency (IF) or a radio frequency (RF) carrier through two separate in-phase (I) and quadrature (Q) channels. Due to the difference between the I and Q channel transmission characteristics (therefore termed I/Q imbalance or mismatch), the signal will be distorted if such impairment exists at the transmitter and/or receiver side(s). If the signal bandwidth is large, the I/Q imbalance is also frequency dependent (i.e., the I/Q imbalance is different at different frequencies throughout the bandwidth).
There are a number of techniques found in the prior art for I/Q imbalance compensation. Most of those techniques deal with I/Q imbalance compensation at the receiver side only, whereas both transmitter and receiver side imbalances exist at the same time in real systems. Estimating and compensating for both transmitter and receiver side imbalance are very challenging as the imbalance signals are entangled and therefore generally complex to separate them to achieve good estimation. Existing approaches typically require offline calibration to obtain the estimate for the transmitter side mismatch, and then estimate the receiver side mismatch using the received signal. However, this calibration will interrupt the normal operation, and is infeasible in continuous transmission systems such as backhaul systems. A limited number of approaches propose to jointly estimate the transmitter and receiver side mismatches, however, their complexity is very high which makes them impractical for implementation in real hardware.
A need therefore exists for alternative equalizers for use in a low latency high speed communication system.